In conventional spectrographs, a single diffraction grating is positioned in the path of the incident electromagnetic radiation. When a different grating is desired, the spectrograph must be at least partially dissembled, the original grating removed and the new grating installed. This is extremely disadvantageous, both in terms of time and inconvenience, but also because of the potential for loss of accuracy due to accidental manipulation of spectrometer elements during the change of gratings.
Multiple diffraction grating spectrographs are known in the prior art, e.g., Spex Industries, Inc. of Edison, N.J. offers "M Series" spectrometers with a dual-grating turret in which gratings are alternately selected by rotating the turret about an axis parallel to the grating line direction.
However, a disadvantage of this design is that changing from one diffraction grating to another requires rotating the grating holder in a direction which changes the angle between incident electromagnetic radiation and the plane of the diffraction grating in a direction perpendicular to the grating lines. This angle is critical to selection of a particular wavelength band for examination by the spectrograph and may need to be reset each time the diffraction grating is changed, making data collection inefficient and tedious.
What is desired, therefore, is a multiple diffraction grating spectrograph which permits changing of the grating without unnecessarily affecting the critical wavelength selecting angle between incident electromagnetic radiation and the plane of the grating.